Permanent closure of the bronchial stump in pneumectomy is essential to a low mortality accompanying that condition. Experimental studies by Bettman, and ourselves, show healing of the stump to be brought about by peribronchial tissue. Thus, whereas the removal of a lung with a generous stump left for closure is usually attended by success, the reverse is often true when a bare bronchial closure remains. The instigation of the following experimental work was the finding of a safe and reliable method for closing large bronchi. This consisted of an application of a 35% solution of silver nitrate to the entire circumference of a bronchus. Complete stenosis followed within 2 weeks. With the bronchus stenosed, the lung became atelectatic and its subsequent removal could be safely and easily accomplished. The following experiments were carried out to determine the maximum amount of pulmonary tissue that could be collapsed or removed in the dog. In the earlier experiments each bronchus was stenosed individually and only one or 2 lobes were removed at a single operation. Later, however, the right or left primary bronchus was dealt with as a whole and the entire lung removed at one stage. Subsequent stenosis of remaining bronchi was attended with some difficulty, this increasing as more of the pulmonary tissue was collapsed. Once the bronchus was completely stenosed, however, its subsequent removal was always attended by success. Subtotal Atelectasis and Subtotal Pneumectomy in 6 Dogs. The percentages in Table I represent the value of each lobe in comparison to the total lung volume by weight. Those on the right represent that portion of the total pulmonary tissue functioning at the present time.